In prior art network analyzers and other two-channel a.c. test instruments, mechanical line stretchers have been used in the reference signal path to eliminate phase differences between the two channels due to different length signal paths. Since mechanical line stretchers require movable contacts which allow line length to change but maintain the same characteristic impedance, they are expensive to build and the various mechanical components wear with use. In addition, such mechanical line stretchers are practical only at higher RF and microwave frequencies. The length necessary for a line stretcher at lower frequencies makes a mechanical line stretcher physically impractical and requires such a large amount of additional transmission line that significant signal losses are often involved.
According to the preferred embodiment of the present invention, the mechanical line stretcher in a network analyzer is replaced by an electronically variable phase shifter. The function of the line stretcher is to introduce a phase shift that varies with the frequency of the test signal. The same function can be implemented by a variable phase shifter which is responsive to the frequency of the signal that the signal generator is supplying. For ease of signal processing, most network analyzers convert the test and reference signals to intermediate frequencies before the phase and amplitude of these signals are measured. The frequency of the intermediate signals does not vary but the phase and amplitude of these signals track the test and reference signals, since the local oscillator used to convert the test and reference signals is also derived from the signal generator driving the device under test. Because it is easier to implement a phase shifter at a single frequency, it is convenient to place the electronic line stretcher in the intermediate frequency portion of the reference channel.
A phase shifter can be implemented in a number of ways, but it is desirable to use a phase shifter that produces a very wide range of phase shifts. Furthermore, it is most convenient to use a phase shifter that is linear in its response to a control signal. A phase lock loop will provide phase shifts over a range of .+-.180.degree. that are linear with respect to an offset voltage added to the error voltage in the control loop. If the signal generator supplying the test signal sweeps over a broad range of frequencies, however, it is usually necessary to have a phase shifter that will supply considerably more than 360.degree. of phase shift, since a signal path length difference that represents only a few degrees of phase shift at a low frequency will represent several hundred degrees of phase shift at a much higher frequency.
Thus, according to the preferred embodiment of the present invention, a phase shift multiplier circuit is provided to multiply the phase shift produced by a phase shifter. Phase shift may be multiplied by multiplying the frequencies of the unshifted and shifted signals and then mixing the multiplied signals to produce a signal of the same frequency as the unshifted signal. When the frequency of a signal is multiplied, the phase shift of that signal with respect to another similarly multiplied signal is also multiplied. When a signal is converted by mixing with another signal, however, phase relationships are preserved because the process is additive. Thus, the multiplication of the two signals multiplies the phase difference between them and the heterodyne frequency conversion preserves that greater phase shift. In this manner, the .+-.180.degree. phase shift produced by a phase lock loop can be multiplied many times.